Increased demands on the agricultural and forestry industries due to world-wide population growth have resulted in efforts to increase plant production and/or size. Although one means for increasing plant size is through plant breeding programs, such breeding programs are typically time-consuming and labor-intensive. Genetic manipulation of plant characteristics through the introduction of exogenous nucleic acids conferring a desirable trait, on the other hand, can be less time-consuming and possibly applicable across a variety of plant species.
Plants produce a number of steroids and sterols, termed brassinosteroids (BRs), some of which function as growth-promoting hormones. There are over 40 BRs known, typically with characteristic oxygen moieties at one or more of the C-2, C-6, C-22, and C-23 positions. Brassinolide (BL) is the most bioactive form of the growth-promoting BRs. Arabidopsis CPD and DWF4 are cytochrome P450 proteins that catalyze enzymatic steps in the BL biosynthetic pathway; they are 43% identical at the amino acid level. During the biosynthesis of BL, DWF4 catalyzes the oxidation of campestanol at C-22 to form 6-deoxocathasterone, while CPD catalyzes the adjacent step downstream, the hydroxylation of 6-deoxocathasterone at C-23 to produce 6-deoxyteasterone.